The invention relates to the design of a stator blade for a turbomachine. In particular, it relates to the design of a vane root to provide a detachable connection between the vane body and the stator casing of such a machine by means of attachment elements, preferably screws, which act with a force fit.
RU 2038487 C1 discloses a widely used type of attachment for the stator vanes to the stator casing of a multistage turbine. The stator, which encloses a conically expanding flow channel, is composed of a plurality of segments. The inner shell of the stator casing has annular recesses, in which projections on the vane root engage and form a positively locking connection between the stator vane and the stator. This solution makes manufacture of the stator casing complex, owing to its complicated structure, characterized by numerous projections and recesses.
In order to avoid these disadvantages, it is known for the stator and stator vane to be connected by means of attachment screws (A. V. Shtoda et al. Konstukcija aviazionnych dwigatjeljei, Zhukovsky Military Aircraft Engineering Academy 1958, p. 141). The stator vane root is equipped with an expanded region in the form of a plate, for this purpose. The vane is attached to the casing by means of two screws. In comparison with that mentioned above, this proposal allows the stator casing to be constructed in a considerably simplified manner, with a correspondingly reduced manufacturing cost, since it is possible to dispense with the complicated internal structure. The gap in the labyrinth seal can be designed to be narrower, since there is no need for the play required, for production engineering, between the mutually engaging projections and recesses on the stator and stator vane.
However, this solution has the disadvantage that two screws are required to fix the vane securely. The root plate is fixed to the stator by one screw upstream of the vane body in the flow direction, and another downstream of it. This additional space requirement for screw arrangement leads, in the end, to the turbine being undesirably lengthened. Temperature differences between the vane root and the stator casing during non-steady-state process phases result in different thermal expansion of the materials. This leads to undesirable material stresses in the area located between the screws. Finally, the complete vane root including the attachment elements is subjected to the influence of the hot process gases, without any protection. This, on the one hand, increases the amount of heat transferred to the stator, while on the other hand uneven areas on the surface cause disturbances in the channel flow.
The invention is based on the object of providing a stator vane which allows a simple and reliable screw connection to the stator, while in the process avoiding said disadvantages of the prior art solutions.
The object is achieved according to the invention by designing a stator vane in accordance with the features of claim 1. Advantageous developments of the invention are described in the dependent claims.
The basic idea of the invention is to design the vane root as a hollow profile, comprising two root platforms which are arranged at a distance from one another, and of which the first, radially inner platform is matched to the conical contour of the flow channel, while the second, radially outer platform is matched to the contour of the corresponding casing wall. Two mutually opposite sidewalls connect the root platforms, enclosing a cavity. The outer root platform is preferably equipped with a hole for accommodating an attachment element, preferably an attachment screw, which fixes the stator vane on the wall of the stator casing.
The cavity enclosed by said root platforms and sidewalls provides space to accommodate the attachment elements, as will be explained in more detail in the following text.
The surface of the outer root plate is provided with recesses. When placed against the stator casing, this creates additional cavities, which make it more difficult for heat to be transferred. Alternatively or cumulatively, the inner wall of the stator casing may also have corresponding recesses.
In one alternative embodiment, the outer root platform is not an integral component of the vane root, but is detachably connected to it. In this case, the vane root is formed only by the inner expanded region, which faces the vane body and is in the form of a plate, and the two radially oriented walls which are arranged opposite and whose ends are bent inward at right angles. The steps produced in this way act as contact edges for the outer plate.
In another alternative embodiment, the sidewalls are mutually connected by two partitions so that two walls, the two partitions, the inner root platform, the outer root platform and the cylindrical surface of the stator casing form a cavity. In this case, if the walls and the partitions are tightly fit against the cylindrical casing surface, and this joint is tightened by a screw connection, a sufficiently sealed connection can be created at this joint.
In this case any possible mass flow values of cooling gas can be supplied into this cavity through a channel made in the casing and through the channel in the outer root platform without worrying about large losses due to its leakage into the flow channel. In this case another channel can be made in both the outer platform and the casing to discharge cooling gas from this cavity.
In the third alternative embodiment, bearing surfaces of each vane are implemented at different radial levels and are coupled with different cylindrical surfaces of the casing; in this case two bearing surfaces of two adjoining vanes are situated on the same cylindrical surface of the stator.
This arrangement of vane bearing surfaces makes it possible to reduce heating of one vane root wall due to its withdrawal from the hot zone interlinked with the flow channel, and to reduce heating of the stator end wall conjugated with the vane wall. In the fourth alternative embodiment, sidewalls are arranged along the direction parallel to the longitudinal axis of the stator with their bearing surfaces adjoined to the cylindrical surface and to the end surface of stator casing. Ends of the root inner platform also adjoin the above mentioned surfaces of the stator casing. The fastening element is arranged obliquely with respect to the cylindrical surface and to the end surface of the stator casing, and oriented for simultaneous pressing of the bearing surfaces of the stator casing. This sufficiently seals the cavity from hot gases passing through the gas flow channel is formed between the end surface of stator casing, its cylindrical surface, the vane root inner platform and its two walls.
This makes it possible to cool internal walls of this cavity by any cooling gas mass flows without misgiving occurrence of large losses due to leakages of coolant into the gas flow channel. In this case an outlet channel for discharge of cooling gas from this cavity can be also implemented in the casing. In so doing cooling of the vane root cavity and even vane airfoil internal cavity (if any) virtually without cooling gas losses, and thus a plant efficiency increase is possible.
An important advantage of this embodiment is also the possibility for vane mounting without any seals between platforms and other root parts of vanes, because proposed design of joints between the vane root part and the stator casing allows for virtually complete protection of inner stator surface against effect of hot gases from the turbine gas flow path.
The advantages of the vane root design according to the invention are, in particular, that the heat transferred from the flow channel to the stator is considerably reduced. The gas-filled cavity enclosed by the hollow profile restricts the passage of heat. The root platform, which rests directly against the stator casing, is subject to less thermal stress. This reduces the thermal conduction both via the contact surface and via the attachment element. Overall, this results in the stator casing temperature being lower. The accommodation of the attachment element or elements in the cavity furthermore protects them against the direct influence of the hot process gases which, not least, also increases the reliability of the connection, and thus safety. Since, in this solution, the attachment element does not occupy any additional space in front of and behind the vane body, the axial extent of the root platform is limited to the size governed by the vane body. Thus the invention does not suffer from the disadvantage of lengthening of the turbines with screwed-on stator vanes, over those with stator vanes that are held with a guided joint.
The root platforms and attachment elements have no effect on the channel flow.
Finally it is feasible to apply a cooling medium deliberately to the cavity, and thus provide additional protection against thermal stress in the stator casing and attachment elements.